1. Field of the Invention
This invention relates to cellular mobile telephone systems in general and, more specifically, to a method of accommodating delays introduced into the mobile telephone system due to preprocessing of the received signals at the base station.
2. Brief Description of the Prior Art
In time division multiple access (TDMA) cellular telephone systems, particularly as used in the United States (IS-54 and IS-136), each mobile telephone is assigned time slots in which to send digitally modulated radio frequency (RF) data bursts. To perform properly, it is vital that the data bursts from the mobile telephone arrive at the base station at the correct time (i.e., in the correct time slots, without overlap into the preceding or succeeding time slots). Several mechanisms are presently in place in the standards and protocols to insure that this problem is avoided.
Two principal prior art methods are in the standards to facilitate the proper timing of the data burst transmissions. First, an equalization/timing sequence (commonly called the sync word) is transmitted in each RF data burst in addition to the data. The receivers at the base station determine the exact time of arrival of the sync word (and hence the entire data burst) by means of correlation of a locally generated sequence (i.e., generated at the base station) with the received sequence. After determining the timing error, the base station sends timing correction information to the mobile telephone. The mobile telephone then corrects its transmit timing. In this manner, the timing error is maintained at an acceptably small value. This mechanism is accomplished to correct for timing advances or delays caused by motion of the mobile user.
The second prior art method in the standards is provided to accommodate the initial transmission of the mobile telephone. At the first transmission of the mobile telephone, there have been no previous timing corrections from the base station, and the mobile telephone has only a coarse estimate of proper transmit timing. Therefore, its initial transmission can have considerable timing error. The timing error in the initial transmissions can be large enough to cause the data burst received at the base station to overlap into preceding or following time slots, thus interfering with other telephone conversations. To prevent this from occurring, the initial transmission is made with a shortened burst containing multiple repetitions of the sync word in a unique sequence. The burst is shortened to allow for greater timing error without the possibility of overlap into adjacent time slots. The unique sequence of sync words is provided so that the base station can determine the arrival time of the data burst without ambiguity and without extensive searching for a received sync word. The base station determines the arrival timing of the initial transmission and sends timing corrections back to the mobile telephone to be implemented as in the previous description. Once initial timing corrections are made, remaining transmissions from the mobile telephone proceed with normal (i.e., not shortened) data bursts. Thus, the initial transmissions of the mobile telephone as well as succeeding transmissions are assured of arriving at the base station totally within the correct time slots. This mechanism is essential to avoid simultaneous arrivals of multiple mobile signals (collisions) at the base station. A diagram of the above described prior art procedure is set forth in FIG. 1.
In some applications, it is desired to preprocess the signals received at the base station in a separate processor prior to feeding the signals to the base station electronics. One such application is known as xe2x80x9csmartxe2x80x9d or signal processing antenna. In this arrangement, the antenna processing is placed in the signal path between the base station antenna elements and the base station electronics signal inputs. The placement of the antenna processing in this position adds delay to the signals, this causing the arrival times of the processed signals at the base station electronics to be in error by the amount of the processing delay, in addition to error of the type discussed above with reference to FIG. 1. Very short preprocessing delays (i.e. a small fraction of a slot time) can be handled as an error and corrected as a part of the error correction as discussed above with regard to FIG. 1. However, if the delay becomes larger (i.e, on the order of one tenth of a slot time or greater), such a large delay cannot be handled by the error correction discussed in conjunction with FIG. 1 and requires a different approach because a large preprocessing delay precludes the base station from making an arrival time measurement. Accordingly, if such delays are permitted to exist, the base station cannot make a correct timing measurement of the type discussed above with reference to FIG. 1 and the system will fail. This problem is depicted by the diagram in FIG. 2.
In accordance with the present invention, there is provided a solution to the problem as described hereinabove without disturbing the normal timing mechanism of the base station or requiring any reprogramming of the base station software.
Briefly, the signalling structure in the mobile telephone system is organized into consecutive, non-overlapping frames, each frame having the same predetermined time duration. Each frame is broken down into a predetermined number of consecutive non-overlapping time slots. The base station receives the RF data bursts from the cellular mobile telephone as in the prior art and the base station electronics measures the times of arrival of the digital data bursts on a time slot basis as in the prior art for each time slot and transmits timing correction data back to each mobile telephone as in the prior art. In other words, a measurement of data burst arrival time is made during each time slot and timing corrections are derived at the base station and transmitted back to the originating mobile telephone.
In order to accomplish the above, it is necessary that the delay introduced by the preprocessing circuitry be compensated for. This is accomplished by providing additional delay circuitry between the preprocessing circuitry and the base station electronics to provide the complement of the delay caused by the preprocessing circuitry and a frame time (i.e, the delay in the preprocessing circuitry plus the delay from the delay circuit is equal to an integral number of frame times). This is shown in FIG. 4. The time delay of the preprocessing circuitry can be fixed, in which case the time delay of the delay circuitry is fixed. The time delay of the preprocessing circuitry can also be variable, changing in response to the signals at the preprocessor inputs, in which case the time delay of the delay circuitry is made variable, preferably on-line. In the case of variable delay, the delay circuitry includes circuitry and/or software to monitor the time delay introduced by the preprocessing circuitry and adjusts the delay in the delay circuit in accordance with the monitored delay in the preprocessing circuitry. The changes in time delay through the preprocessing circuitry can be caused by code accompanying a change in the data entering the base station antenna (i.e., in response to the type of signal (e.g., noisy, not noisy, etc.) entering the base station antenna). Thus, the burst transmission of a given mobile telephone arrives at the base station electronics in the proper slot and with the proper timing relative to the slot, albeit with a time delay of an integral number of full frames (i.e., one or more). Since the base station makes measurements on a time slot basis, the base station is unaware of the delay and computes the timing corrections as if there were no additional delay, which is the desired result.
It can be seen that, in accordance with the circuitry and procedure as described above, preprocessing with built in delay can be provided without disturbing the normal timing mechanism of the base station or reprogramming of the base station software. The system continues to operate as in the prior art with timing correction available and in use as described above in connection with FIG. 1.